Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create a beam of positively charged ions, which is then directed toward the workpiece. As the ions strike the workpiece, they change the properties of the workpiece in the area of impact. This change allows that particular region of the workpiece to be properly “doped”. The configuration of doped regions defines the functionality of the workpiece, and through the use of conductive interconnects, these workpieces can be transformed into complex circuits.
In many applications, the ion beam is directed so as to strike the workpiece in a direction normal to the plane of the workpiece. FIG. 1 shows a representative orientation of workpiece 100 in three dimensions, X, Y and Z. In many applications, the ion beam (not shown) is directed toward the workpiece 100 in a direction substantially parallel to the Z axis. In this manner, the ion beam is substantially perpendicular to the workpiece in both the X and Y axes. The ion beam can be thought of as a set of ion beamlets, each beamlet comprising a single line in the XZ plane. While it is important that the entire beam is substantially perpendicular to the workpiece, it is equally important that each of the individual beamlets is also perpendicular to the workpiece in both the X and Y axes. FIG. 2a shows an ion beam 200 which is made up of a plurality of beamlets 210. Although only several are shown for purposes of illustration, the ion beam can be comprised of an arbitrary number of beamlets. In this Figure, all of the beamlets 210 are parallel to one another. In contrast, FIG. 2b shows an ion beam 250, which is also substantially perpendicular to the workpiece. However, its component ion beamlets 270 are not parallel to one another, and consequently some of these beamlets are not perpendicular to the workpiece.
Since an ion beam is a three dimensional entity, parallelism exists in several dimensions. For example, ion beamlets can be parallel across the X dimension (i.e. the XZ plane). In this dimension, deviations in the Y dimension are not considered. Similarly, ion beamlets can be parallel across the Y dimension (i.e. YZ plane), where deviations in the X dimension are not considered. It should be obvious to one of ordinary skill in the art that the ion beamlets of an ion beam can display parallelism in one dimension (i.e. width) without displaying parallelism in the orthogonal dimension (i.e. height). While this issue is well known in the art, it is not considered important, since the mechanical movement of the workpiece support ensures that all portions of the workpiece are exposed to the beam.
U.S. Pat. No. 6,437,350, which is incorporated herein by reference, discloses an apparatus and method of maintaining the desired degree of parallelism between the ion beamlets. Those of ordinary skill in the art will appreciate that parallelism is typically controlled via an angle corrector, such as a magnet. However, the optimal setting for beamlet parallelism may occur at an incident angle other than 90°. To correct for this, the workpiece is tilted about a line parallel to the Y axis. As shown in FIG. 3, the ion beam exits the angle corrector 330. Because of the magnetic fields created by the angle corrector 330, the beamlets 310 of ion beam 300 are parallel to one another. However, their angle of incidence is not orthogonal to the workpiece 320. To rectify this, workpiece 320 is pivoted about a line at an angle 340. In this way, the ion beam 300 strikes the workpiece 320 perpendicularly.
Certain applications require that the ion beam strike the workpiece with an incident angle other than 90°, such as at large tilt angles. In one embodiment, high tilt implantation, such as halo or pocket implantation, is used to create dopant pockets underneath the edge of the gate device. FIG. 4 shows a representative transistor structure 400, comprising a gate region 410, atop a substrate 420. The source region 430 and drain region 440 lie on either side of the gate region 410. To extend the source and drain regions beneath the gate edge, the ion beam impacts the substrate at a significant tilt angle, as demonstrated by directional arrows 450a and 450b. This tilt angle is typically between about 5 and about 60 degrees.
This high tilt implantation is achieved by manipulating the workpiece in several dimensions. FIG. 5 demonstrates a preferred method in the prior art of performing these steps. FIG. 5 shows workpiece 500, which is substantially aligned with the XY plane. The ion beam 510 is substantially parallel to the Z axis, and is perpendicular to the workpiece 500. As described above, the workpiece 500 is pivoted about a line parallel to the Y axis to insure that the ion beam strikes the workpiece at a 90° angle. To achieve the high tilt implantation, the workpiece is then pivoted about a line that is parallel to the X axis. The workpiece is then scanned by the ion beam 510 in the traditional manner. Traditionally, the beam is scanned back and forth in the x direction either electrostatically or by using a magnet, and the workpiece is translated in the y direction relative to the beam. The magnetic or electrostatic scan is usually referred to as fast scan, the mechanical scan as slow scan. Alternatives include replacing the fast scan with a ribbon beam, and replacing the fast electrostatic or magnetic scan with a mechanical scan (2D mechanical scanning). When the workpiece has been scanned, the workpiece has been implanted at a high tilt angle, such as 450a as shown in FIG. 4.
To implant ions at the incident angle represented by 450b as shown in FIG. 4, it is necessary to rotate the workpiece again. In this case, the workpiece is preferably rotated about a line that is perpendicular to the surface of the workpiece. A rotation of 180° is needed to generate the incident angle represented by 450b. 
This combination of movements about three separate axes allows the workpiece to be implanted at high tilt angles. Despite the precision movements described above, it is difficult to achieve angular precision within ±0.5 degrees. Thus, an implantation angle of 60° may actually be between 59.5° and 60.5°. As the geometries continue to decrease in size, it is becoming more and more important to have precise control over each of these angles, especially the angle of incidence. Thus, the amount of deviation in conventional processes is becoming unacceptably high.